The invention relates to a method of manufacturing electronic devices, in particular, but not exclusively, semiconductor devices, in which method substrates, which are provided at a surface with a silicon oxide-containing material which is to be remove later, are subjected, while being divided into successive batches, to a wet treatment in a bath containing a solution of hydrofluoric acid in water, during which wet treatment the conductivity of the solution is monitored and the silicon oxide-containing material is removed, thereby forming ionic components, which monitored conductivity is brought to approximately a desired conductivity at time intervals by adding hydrofluoric acid and/or water to the solution inside the bath.
An important wet treatment in IC manufacturing technology involves wet etching of silicon oxide-containing material, which is frequently carried out in a bath containing a solution of hydrofluoric acid in water (HF/H2O). Consumption of fluoride containing components and formation of reaction products as a result of the removal of the silicon oxide-containing material, evaporation of water and/or hydrofluoric acid, and dragging into/out of water and chemicals when the substrates are, respectively, placed in/removed from the bath, are important factors influencing the chemical composition of the hydrofluoric acid solution over time. As the rate of etching or removal of the silicon oxide-containing material depends inter alia on the chemical composition of the hydrofluoric acid solution, it is important to monitor and control the chemical composition of the hydrofluoric acid solution so as to reach a better control of the etch process. An important category of techniques applied to monitor and control the chemical composition of solutions is based on the measurement of the solution""s conductivity.
A method of the kind mentioned in the opening paragraph is known from an article entitled xe2x80x9cIn-situ chemical concentration control for substrate wet cleaningxe2x80x9d, written by Ismail Kashkoush et al. and published in Mat. Res. Soc. Symp. Proc., Vol. 477 (1997), pp. 311-316.
It has been experimentally observed that, although the conductivity of the hydrofluoric acid solution inside the bath is accurately controlled by bringing the monitored conductivity to approximately the desired conductivity at the time intervals, a decrease in removal or etch rate of the silicon oxide-containing material takes place from batch to batch during the wet treatment of the successive batches of substrates. In this way, the process stability of the wet treatment of the successive batches deteriorates, and the lifetime of the hydrofluoric acid solution is shortened.
It is an object of the invention to provide a method of the kind mentioned in the opening paragraph, which method improves the process stability of the wet treatment of successive batches of substrates, which are provided at a surface with a silicon oxide-containing material which is to be removed later, said wet treatment being carried out in a bath containing a solution of hydrofluoric acid in water, and lengthens the lifetime of the solution.
According to the invention, this object is achieved in that prior to the wet treatment the solution is provided with a basic component MX so that the monitored conductivity of the solution decreases as a result of the removal of the silicon oxide-containing material in between the time intervals of the wet treatment, the basic component MX comprising a cation M(+), which is different from H+, and an anion X(xe2x88x92), which anion is a conjugated base of a weak acid.
The invention is based inter alia on the insight that the increase of the conductivity owing to the ionic components formed in the reaction between the hydrofluoric acid and the silicon oxide-containing material present at the surface of the substrates is larger than the decrease of the conductivity owing to the fluoride-containing ionic components consumed in this reaction. Hence, as a result of the removal of the silicon oxide-containing material in between the time intervals of the wet treatment, the conductivity of the solution increases. Simultaneously, the etch rate of the silicon oxide-containing material decreases owing to the consumption of hydrofluoric acid. Based on the monitored conductivity, water is added to the solution to bring the conductivity to approximately the desired value, whereas actually hydrofluoric acid should be added to bring the etch rate to approximately the desired value. Because of the addition of water to the solution, the concentration of hydrofluotic acid in the solution decreases further, which in turn results in a further decline of the etch rate of the silicon oxide-containing material. This decline in etch rate can be counteracted by the addition of a basic component MX to the solution prior to the wet treatment of the successive batches of substrates, thus causing the increase of the conductivity owing to the ionic components formed as a result of the removal of the silicon oxide-containing material to be smaller than the decrease of the conductivity owing to the fluoride-containing ionic components consumed as a result of this removal. The basic component MX should comprise a cation M(+), which is different from H+, and an anion X(xe2x88x92), the anion being a conjugated base of a weak acid, which means that, if the basic component MX is added to water, the anion of the component may react with a proton to form HX. Using the measure in accordance with the invention, the conductivity of the solution changes in the same direction in between the time intervals of the wet treatment as the etch rate of the silicon oxide-containing material does. As a consequence, addition of hydrofluoric acid is required on the basis of conductivity as well as on the basis of etch rate. In this way, the process stability of the wet treatment is improved, and the lifetime of the solution is lengthened.
For reasons of adequacy, the basic component is advantageously added to the solution in an amount which is determined on the basis of the amount of silicon oxide containing material to be removed during the wet treatment of the successive batches of substrates. A further improvement of the process stability of the wet treatment and the lifetime of the solution can be accomplished by adding, apart from hydrofluoric acid, a further basic component MX to the solution at the time intervals, so that in addition the etch rate is brought to approximately a desired etch rate at the time intervals, the further basic component MX comprising a cation M(+), which is different from H+, and an anion X(xe2x88x92), which anion is a conjugated base of a weak acid.
For reasons of adequacy, the hydrofluoric acid and the further basic component are added to the solution at the time intervals in a ratio which is determined on the basis of the composition of the solution adjusted prior to the wet treatment of the successive batches of substrates and the amount of silicon oxide-containing material to be removed during the wet treatment of the successive batches of substrates.
In order to reduce the complexity of the process, it is advantageous to apply one and the same component as the basic component, which is added to the solution prior to the wet treatment, and as the further basic component, which is added to the solution at the time intervals.
As the effect on the monitored conductivity of the ionic components formed as a result of the removal of the silicon oxide-containing material from the surface of the substrates is more pronounced for dilute solutions, the method in accordance with the invention is more effective if the solution of hydrofluoric acid in water is applied as a dilute solution. Moreover, as a result of the continued miniaturization of semiconductor devices, dilute solutions of hydrofluoric acid in water are becoming increasingly important in IC manufacturing technology. Dilute solutions with a weight percentage (wt. %) HF in the range from about 0.001 to 5.0 are advantageously applied for the wet etching of thin silicon oxide containing layers, that is to say silicon oxide-containing layers having a thickness of a few nanometers up to several tens of nanometers.
Further advantageous embodiments of the method in accordance with the invention are described in other dependent claims.
The invention further relates to an apparatus for carrying out the method as claimed in any one of the preceding claims, the apparatus comprising a bath containing the solution and accommodating the substrates, supply means for supplying the hydrofluoric acid and the water to the bath, means for monitoring the conductivity of the solution, and control means for bringing the monitored conductivity to approximately the desired conductivity at the time intervals.
Such an apparatus is known from an article entitled xe2x80x9cIn-situ chemical concentration control for substrate wet cleaningxe2x80x9d, written by Ismail Kashkoush et al., published in Mat. Res. Soc. Symp. Proc., Vol. 477 (1997), pp. 311-316.
In order to improve the process stability of the wet treatment of successive batches of substrates, which are provided at a surface with a silicon oxide-containing material to be removed, in a bath containing a solution of hydrofluoric acid in water, and to lengthen the lifetime of the solution, the apparatus in accordance with the invention is characterized in that it comprises further supply means for supplying the basic component to the bath.